1. Field of the Invention
This invention relates to improvements in circuits and methods for providing a programmable differential current output from a differential voltage input, and more particularly to improvements in programmable differential transconductance circuits.
2. Relevant Information
Transconductance circuits are circuits that change a voltage into a current, and have many uses. For example, transconductance circuits are used in motor driver applications, such as for hard disk drives, or the like, or for filter applications, and so on. Typically, in filter applications, one of the design considerations is the ability to programmably adjust the range of frequencies that are passed or rejected by the filter, which can be controlled by varying the currents applied to the filter in known manner.
Generally the currents that are applied to the filter are provided by transconductance circuits. However, because of limitations in the transconductance circuit designs, the frequency range over which the filter could be adjusted has been limited to about three times the center or cutoff frequency. This limitation has primarily been due to the range over which the transconductance circuit associated with the filter can be reliably operated.
One transconductance circuit which has been used employs a resistor connected between emitters of bipolar transistors that are connected in respective current steering paths. The resistor is sometimes referred to as a "degenerating resistor." An input control voltage controls the current steering paths to direct current between output current paths and a supply current path. The input signal is applied to the respective bases of the bipolar transistors in the current steering paths. One of the problems in the construction of this type of transconductance circuit, however, is that the particular value of the resistor used to interconnect the steering paths is critical. When an integrated circuit implementation is used, the value of the resistor is usually adjusted by laser trimming techniques, in order to overcome the typical integrated circuit processing variations, which may be as high as .+-.30%. Laser trimming, however, requires expensive equipment to perform, is time consuming, must be performed on the resistors of each transconductance circuit, and adds greatly to the final cost of the circuit.
Not only do resistor embodiments require laser trimming and other expensive procedures, but resistors used in integrated circuits in the transconductance steering current paths typically are also undesirably temperature dependent. For example, a typical resistor in the transconductance circuit environment of the type described may have a temperature coefficient of up to .+-.1%/.degree. C. Thus, over a large temperature range which may typically be encountered in an environment in which transconductance circuits and their associated circuitry may be employed renders such resistor embodiments generally unsatisfactory for use.
To address these problems, circuits have been suggested that employ a number of input stages, with each stage being switchably connected to the main transconductance circuit. The circuits include multiple MOS devices interconnecting the various steering current paths, the various steering current paths being connected in parallel so that the binary sum of the individual contributions of the MOS transistors provides the necessary current control, as determined by the circuit switches.
This embodiment, however, has several limitations, including the resolution that is achievable and, in general, the current range over which the transconductance operation of the circuit can perform. In a filter embodiment, for example, this results in a limitation on the range of frequencies over which the filter can be programmably controlled.
In this embodiment, the MOS transistors are generally operated in a linear operating range between the respective current steering paths in which they are connected. The voltage on the gate of the respective MOS transistors is controlled by a circuit that makes the transconductance of the circuit utilizing this linear region MOS transistor to be relatively temperature independent, but as indicated, the range over which the transconductance current can be controlled is not as great as might be desired in many applications. An example of a circuit for providing a control voltage for application to the gates of the various MOS transistors is shown in U.S. Ser. No. 08/430,218, assigned to the assignee hereof, and incorporated herein by reference.
Another problem of transconductance circuits of this type is that they occupy a large area in an integrated circuit, which can be undesirable in many applications. In addition, because of the number of individual circuits that are switched into and out of the current steering paths, the offsets that are generated in the circuit can be problematic.
What is needed, therefore, is a method and apparatus for providing a programmable transconductance that has a wide range of transconductance, requires little integrated circuit "real estate", and is process and temperature independent.